The present invention relates to a combustion exhaust gas treatment process and apparatus.
Exhaust gas treatment apparatuses for thermal power boilers are commonly equipped with denitration apparatuses designed to remove nitrogen oxides (NOx), electric dust collectors designed to remove dust, and desulfurization apparatuses designed to remove sulfur oxides (mainly SO2).
In cases where the thermal power boiler employs high-sulfur-containing heavy oils and petroleum coke as fuels, it is known that the exhaust gas contains SO2 as well as SO3 and/or sulfuric acid mist. The SO3 and/or sulfuric acid mist contained in the exhaust gas cannot be easily removed with a desulfurization apparatus using calcium carbonate, and their removal has therefore been accomplished by injecting ammonia into the downstream exhaust gas of the denitration apparatus to convert it into harmless ammonium sulfate or ammonium hydrogen sulfate (hereunder also referred to as xe2x80x9cacid ammonium sulfatexe2x80x9d), which is then separated and removed with an electric dust collector and desulfurization apparatus.
The ammonia supplied to the downstream end of the denitration apparatus must be supplied in excess of the stoichiometric amount for complete removal of the SO3 and/or sulfuric acid mist, and the unreacted ammonia supplied in excess is absorbed in the desulfurization apparatus at a later stage. Most of the ammonium sulfate or acid ammonium sulfate, as a reaction product of the SO3 and/or sulfuric acid mist and ammonia, is removed by the electric dust collector at a later stage, and the unremoved ammonium sulfate or acid ammonium sulfate is absorbed at the desulfurization apparatus together with the unreacted ammonia. Thus, the waste water discharged from the desulfurization apparatus contains an abundant nitrogen portion in the form of ammonia, ammonium sulfate and acid ammonium sulfate, and without treatment, such waste water cannot be disposed of under the recently enforced waste water regulations.
The dust ash that has been separated and removed by the electric dust collector also contains ammonium sulfate and acid ammonium sulfate, and since the waste water used to wash this dust ash contains an abundant nitrogen portion in the form of ammonia, ammonium sulfate and acid ammonium sulfate, this waste water also cannot be disposed of under the current nitrogen waste water regulations, unless it is treated. It has therefore been the practice to reduce the nitrogen content of waste water to within regulation standards using a method in which the nitrogen portion is removed through denitration by biological treatment using nitrifying bacteria in the waste water treatment apparatus. However, the method of using nitrifying bacteria for biological treatment of the nitrogen portion dissolved in waste water poses the problem of a large treatment apparatus volume and increased equipment costs, making it necessary to reduce the amount of ammonia added for removal of the SO3 and/or sulfuric acid mist.
In order to reduce the amount of ammonia used, it is desirable to recover and recycle ammonia from the ammonia-containing waste water of the ammonia-containing desulfurization apparatus and the ammonia-containing waste water used to wash the dust ash. However, after separation of the calcium sulfate produced upon desulfurization by the limestone-gypsum method employing calcium carbonate, the filtrate has a high Ca content, and when this waste water is introduced into an air heater or stripper for the ammonia recovery procedure, Ca precipitates in and blocks the evaporator or stripper, causing a problem which is prohibitive to prolonged continuous operation.
It has therefore become a common procedure that after the gypsum (CaSO4) produced in the desulfurization waste water is separated by a filter, the pH is adjusted with an alkali metal hydroxide such as NaOH, and the Ca portion in the filtrate is precipitated and removed together with a coagulant. Still, the filtrate to which the coagulant has been added after pH adjustment for precipitation and separation usually contains at least 200 ppm of Ca, most of which is in the form of Ca(OH)2. When this solution is supplied to an evaporator or stripper for heating, the trace amount of ammonium carbonate included therein releases carbonic acid, and the released carbonic acid reacts with the Ca(OH)2 to precipitate calcium carbonate, thus creating a problem which prohibits prolonged continuous operation.
It is an object of the present invention, which has been accomplished under these circumstances, to provide a combustion exhaust gas treatment process and apparatus, employing a process for treatment of combustion exhaust gas in which the ammonia contained in waste water obtained after desulfurization or waste water used to wash dust ash is recovered, and the recovered ammonia is added to the denitrated gas.
As a result of diligent research aimed at achieving this object, the present inventors succeeded in completing the present invention upon finding that the aforementioned object can be achieved by removing ammonia from a supernatant obtained by filtering a solution which has been desulfurized using an aqueous solution containing calcium carbonate, adjusting the pH of the filtrate to 9-12 while simultaneously adding carbon dioxide and/or an aqueous carbonic acid solution, or else adjusting the pH to 9-12 after adding carbon dioxide and/or an aqueous carbonic acid solution, and then adding a flocculating agent to precipitate and separate a solid portion from the supernatant.
Thus, the present invention provides a combustion exhaust gas treatment process which comprises adding ammonia to denitrated combustion exhaust gas to convert S03 and/or sulfuric acid mist to ammonium sulfate and/or ammonium hydrogen sulfate, removing the produced ammonium sulfate and/or ammonium hydrogen sulfate with dust, desulfurizing the resulting gas using an aqueous solution containing calcium carbonate and filtering the solution to separate calcium sulfate and filtrate, and then adjusting the pH of the filtrate to 9-12 while simultaneously adding carbon dioxide and/or an aqueous carbonic acid solution, or else adjusting the pH to 9-12 after adding carbon dioxide and/or an aqueous carbonic acid solution, adding a coagulant to precipitate and separate a solid portion to form a supernatant, conducting the supernatant to a separately provided ammonia recovery step at which ammonia is recovered by introducing steam for distillation and concentration, and thereafter adding the recovered ammonia to the aforementioned denitrated gas.
The invention further provides a combustion exhaust gas treatment apparatus comprising at least an electric dust collector for removal of dust, a desulfurization apparatus employing calcium carbonate and a waste water treatment apparatus for treatment of the desulfurized waste water discharged from the desulfurization apparatus, the combustion exhaust gas treatment apparatus being constructed in such a manner that ammonia recovered from the desulfurized waste water using a waste water treatment apparatus comprising the following apparatuses (1) to (4) returns to the upstream end of the electric dust collector.
(1) A filtering apparatus for filtration of calcium sulfate.
(2) A pH adjusting apparatus provided with a function of supplying carbon dioxide and/or an aqueous carbonic acid solution.
(3) A precipitation and separation apparatus which precipitates and separates the solid portion after addition of the coagulant.
(4) A concentration apparatus which introduces steam into the supernatant obtained by precipitation and separation of the solid portion, and recovers the ammonia.